Fat fin - a leathery fold without rays on the caudal stem behind the dorsal fin in salmonids, whitefish, smelt, grayling, some carp, catfish and other fish. It consists of fatty tissue and has no true fin rays. It has a rounded shape and is usually much smaller than the dorsal fin, although in some species it reaches a significant development.

The adipose fin is present in eight modern fish orders, including characiniformes, catfishes, and salmonids. Although it is not present in all members of these orders, it is not found in other fishes. The presence of an adipose fin can be traced back to the Mesozoic in fossil fishes (some species considered to be ancestors of salmonids).

The purpose of this organ remains unclear despite ongoing research. Some sources suggest that it serves as an additional store of fat. Because in some species (e.g., seals) the adipose fin of males is larger than that of females, it has been suggested that its size may be an additional factor in attracting females during the breeding season. However, this issue is not fully understood.

A special study by Canadian scientists conducted in 2004 on juvenile steelhead salmon showed that the fat fin may be important for improving the hydrodynamic properties of the fish body. Salmon fry with removed or trimmed adipose fins made movements of the tail part of the body with increased amplitude when swimming. Apparently, the adipose fin reduces the formation of swirls of the incoming water flow behind the dorsal fin. It is also likely that the richly innervated coverings of the adipose fin help the fish to sense the formation of eddies and choose the optimal mode of movement.